1. Field of the Invention:
This invention relates generally to a process for conserving calcium values for reuse in water and wastewater treatment systems and, more particularly, to an improved process for reclaiming calcium values from the solid residuals produced by the incineration or calcining of calcium-bearing sludges.
2. State of the Art:
In water and wastewater treatment processes sludges are often produced that contain calcium-bearing compounds in substantial quantities. The calcium-bearing sludges are formed, for example, in tertiary sewage treatment processes wherein phosphorus is removed from wastewater by lime precipitation. In such sludges, the phosphorous values appear as solid compounds which are generally classified as "apatites" by workers in the art and which can be generally designated Ca.sub.x (PO.sub.4).sub.y where x and y are integers. As another example, calcium-bearing sludges are produced when lime and/or slaked lime are added to raw sewage to serve as a flocculating agent or to water to produce chemical softening; in such cases, the sludges usually include substantial quantities of calcium carbonate. Calcium-bearing sludges are found also in some sugar beet refining operations and in some high-turbidity water treatment systems.
It is well known that the organic materials in such sludges can be disposed of by incineration. Incineration reduces the organic materials to inert ash and concomitantly produces gases and vapors, principally carbon dioxide and steam. It is also well known that sewage sludges can be calcined under controlled conditions to form calcium oxide solids from the calcium carbonate values. In the case of phosphorous-bearing sewage sludges, the apatites which remain after incineration are intimately mixed with the ash and calcium oxide values.
Workers in the field of water and wastewater treatment have long felt a need for an economical system for reclaiming lime values from the solid residuals of incinerated calcium-bearing sludges. If the lime values could be inexpensively reclaimed and recycled in a relatively concentrated form, plant operating costs could be reduced insofar as expenditures for the purchase of lime or limestone are concerned.
According to one previously proposed reclamation technique, the solid residuals of incineration of a calcium-bearing sludge are mixed with water to dissolve the calcium oxide values and to form a sediment of the insoluble components, such as ash. Because calcium oxide is relatively insoluble (its practical solubility is about 1200 mg/l) such a reclamation technique is uneconomical for recovering high percentages of the available calcium. For example, the reclamation of 90-95% of the available calcium values in a sewage treatment plant by means of a dissolving-settling technique would require the handling and recycling of water volumes that equaled about 20-25% of the total plant hydraulic capacity and that, in turn, would require large unit operations and would increase plant capital costs.
Another previously proposed reclamation technique involved a size or density classification of the solid residuals of incineration premised on the fact that calcined lime particles are typically finer and less dense than the solid inert constituents of the incinerated sludge. However, in attempting to recover large fractions (over 70%) of the available calcium values ("accepts") by this classification technique, it was found that size and density overlap caused a progressively increasing proportion of the accepts stream to consist of ash and other undesirable components. In other words, the calcium purity of the accepts stream decreased rapidly as one attempted to recover and recycle larger and larger percentages of the total available calcium. The inclusion of ash and other inerts in the accepts stream increased the cost and decreased the efficiency of the recycle. Furthermore, the efficiency of such a classification system is difficult to predict since the particle size distributions may vary substantially from one geographical area to another.